The power which is generated in electrical grids must equal the demanded power. If equilibrium is suddenly lost, by consumers being switched off or on, grids being disconnected or electricity generating installations being switched off, this loss of equilibrium must be compensated by suitable measures. If very major changes occur suddenly, this places stringent demands on the grid regulation and individual regulation of the individual electricity generating installations. In particular, major consumers or entire grid elements being switched off represents a hazard to the electricity generating installations. When the torque of the consumers is disconnected from the electricity generating installations by circuit breakers being opened, the rotating electricity generating installations start to accelerate, corresponding to their intrinsic mass moments of inertia. If no suitable measures are taken, the electricity generating installations accelerate to their maximum permissible rotation speed, and are switched off by the safety devices.
Systems which prevent impermissible overspeeding after load shedding in an electrical grid are known, for example, from the document US-A-3 601 617. The methods proposed therein use a rotation speed measurement and the rotation speed change derived therefrom as a main variable. As soon as the rotation speed and the acceleration linked to it have exceeded an installation-typical value, measures are introduced to reduce the drive power. In order to detect an increase in rotation speed correctly as a consequence of consumer-end load shedding in an electrical grid, further process variables such as temperature, pressure, flow etc. as well as electrical variables such as power, voltage, current etc. are measured and are included in the evaluation logic. Furthermore, the switches in the electrical grid are also monitored and are included in the logic. The critical factor is that this prior art cannot dispense with the “mechanical” rotation speed measurement as a substantial variable.
These known measures and methods have the disadvantage that reaction is impossible before a certain amount of time has passed after the event that took place in the electrical grid, specifically when the secondary effects of the consumer-end load shedding in the electrical grid can be measured and evaluated in the form of the changing rotation speed and the acceleration in the electricity generating installation. It is therefore impossible to initiate suitable measures at an early stage, and to switch the electricity generating installations to a safe operating state, matched to the situation. The reaction of the electricity generating installations, which is therefore (too) late in consequence leads to the electricity generating installations being switched off in a partially uncontrolled manner, not matched to the situation, with partially uncontrolled effects on the electrical grids (“blackouts”) linked to high financial losses (production failure, wear).
A further method and an apparatus for detection of load shedding are known from the document DE-A1-103 28 932. In this document, the principle for detection of load shedding in the electrical grid is to measure the (alternating) frequency of the AC voltage produced by the generator. This is compared with a predetermined maximum frequency. The acceleration is derived as an additional variable by detecting the frequency over time. This is compared with a predetermined maximum acceleration value. Additional items also take account of:
a) the power (load) of the rotating machine at the time of load shedding for calculation of the load-dependent maximum acceleration value,
b) the state of the rotating machine below or above the minimum electrical power, and
c) the connection and disconnection of the rotating machine to and from the supply grid.
The frequency of the AC voltage produced by the generator is directly proportional to the rotation speed of the rotor of the rotating machine. It is therefore completely irrelevant whether the rotation speed of the rotor of the rotating machine or the frequency of the AC voltage produced by the generator is measured. This means that the rate of change of both variables depends equally on the inertia of the rotor of the rotating machine, and is therefore a “slow” variable. This “dependency on the inertia of rotating masses” represents a major time disadvantage for early or “anticipatory” detection of load shedding in the electrical grid. Furthermore, various disturbances in the grid, such as transient processes, can greatly corrupt a frequency measurement, and the methods based thereon therefore lead to incorrect conclusions.